I. Field of the Invention
The present invention relates to a machine tool for machining a workpiece by feeding a cutting tool into that workpiece in a series of discrete steps and related method.
II. Background Information
In general, in a machine tool drilling operation, a cutting tool is feed into a workpiece in a series of discrete steps. During each step of feeding the cutting tool into the workpiece, chips of workpiece material are removed. At the end of each of the steps, the cutting tool is at least partially withdrawn from the workpiece to cool the cutting tool with cutting oil.
It becomes more difficult to eject or remove chips as the cutting tool is feed deeper and deeper into the workpiece. Moreover, as the cutting tool penetrates into the workpiece, it is more difficult for the cutting oil to reach the cutting tool. Accordingly, as the cutting tool is fed into the workpiece, the rate of feeding of the cutting tool into the workpiece should be gradually decreased.
The prior art recognizes the need to reduce the rate of cutting tool feeding as that feeding progresses into the workpiece. For example, in Japanese patent application Laid-Open No. 223508/1983, the first discrete step of feeding a cutting tool into a workpiece or at least the first several such steps feed the cutting tool into the workpiece a predetermined distance each step. Thereafter, the distance the cutting tool is feed into the workpiece each subsequent step is reduced in a geometric ratio. However, this method of reducing the rate of feeding a cutting tool into a workpiece has several disadvatages. First, special knowledge is required to establish the predetermined feed distance for the first several steps. Second, since the cutting tool is fed into the workpiece a distance which varies as a function of overall feeding depth, when a large diameter cutting tool is employed that has a high strength, the number of discrete steps required to complete an operation is greater than the minimum number of steps ideally needed for that particular cutting tool.
Accordingly, work efficiency is lower than optimum.